For using sonic vibrations to produce a radially uniform resistance characteristic in a semiconductor crystal

ABSTRACT

A method and apparatus for producing a radially uniform resistance characteristic in a semiconductor crystal rod wherein the crystal rod is subjected to a floating melt zone purification and is simultaneously charged with sonic oscillations in its axial direction. The sonic oscillations are generated by an electromagnetic sound generator means and transmitted to the melt zone by a column of liquid or metal interconnecting the generator means with the crystal rod.

United States Patent 1191 Keller Oct. 15, 1974 FOR USING SONIC VIBRATIONS T0 3,206,286 9/1965 Bennett .1 23/273 SP RO U E A RADIALLY UNIFORM 3,216,805 11/1965 Emeis 23/273 SP RESISTANCE CHARACTERISTIC IN A xgrth 23/273 SP ams 23/273 SP SEMICONDUCTOR CRYSTAL 3,592,937 7/1971 15 315.. 23/273 SP 75 Inventor; Wolfgang Keller, Munich, Germany 3,637,439 l/1972 DeB1e.. 23/273 SP [73] Assignee: Siemens Aktiengesellschaft, Berlin primary Examiner Norman Y dk ff and Mumch Germany Assistant Examiner-F. Sever 22 Filed; 4 972 Attorney, Agent, or FirmHill, Gross, Simpson, Van

Santen, Steadman, Chiara & Simpson [21] App]. No.: 283,320

[57 ABSTRACT Foreign Application Priority Data A method and apparatus for producing a radially uni- Aug. 27, 1971 Germany 2143112 form resistance characteristic in a semiconductor crystal rod wherein the crystal rod is subjected to a float- [52] US. Cl. 23/301 SP, 23/273 SP ing melt zone purification and is simultaneously [51] Int. Cl B0lj 17/00 charged with sonic oscillations in its axial direction. [5 8] Field of Search 23/301 SP, 273 SP, 273 F The sonic oscillations are generated by an electromagnetic sound generator means and transmitted to the [56] References Cited melt zone by a column of liquid or metal interconnect- UN STATES PATENTS ing the generator means with the crystal rod. 2,952,722 9/1960 Jackson 23/273 SP 11 Claims, 2 Drawing Figures N 1L 1 E m 3 12 5 g ,Qq Eia FOR USING SONIC VIBRATIONS TO PRODUCE A RADIALLY UNIFORM RESISTANCE CHARACTERISTIC IN A SEMICONDUCTOR CRYSTAL BACKGROUND OF THE INVENTION l. Field of the Invention The invention relates to production of semiconductor crystals and more particularly to a method and apparatus of producing a radially uniform resistance characteristic in a semiconductor crystal.

2. Prior Art Production of monocrystalline semiconductor rods by floating melt zone techniques is known. Generally, polycrystalline semiconductor rods, such as of silicon, are transformed into single or monocrystalline structures by placing a seed crystal at one end of the polycrystalline stock rod and establishing an annular melt zone at such end and then slowly moving the melt zone along the rod to the other end thereof. The resolidified portion attains a higher degree of crystalline order and forms a single crystal. In such arrangments, the semiconductor rod is generally vertically mounted between a pair of movable mounting means, one of which rotates about the axis of the rod during the movement of the melt zone, assuring a symmetrical growth of the resolidified portions.

Single crystal rods produced by floating melt zone techniques exhibit a radially variable resistance characteristic, apparently because of insufficient mixing within the melt zone thereof. Of course, it is highly desirable in the production of semiconductor components that the semiconductor crystal have a radially uniform resistance characteristic.

German Letters Pat. No. 1,218,404 suggests that an eccentrically floating melt zone be utilized about semiconductor crystals to attain improved mixing of the melt. Such modified floating melt zone technique is said to improve the uniformity of the radial resistance characteristics in resolidified portions of rods treated thereby. In accordance with this teaching, the rotating mounting means at the resolidified-crystal portion is shifted or located at an angle relative to the heating means, i.e., the induction heating coil. Such an arrangement is fairly complex and does not yield reproduceable results.

SUMMARY OF THE INVENTION The invention provides a novel method and apparatus for producing a semiconductor crystal characterized by a radially uniform resistance characteristic.

It is a novel feature of the invention to subject a semiconductor crystal to a floating melt zone purification process and to subject such crystal to axially directed sonic oscillations.

It is another novel feature of the invention to interconnect the melt zone of a semiconductor crystal with a means for generating sonic oscillations by a sound transmission means, such as a liquid or metal body.

It is a further novel feature of the invention to utilize a magnetic coil means that is energized by an altemating current to produce sonic oscillations in a melt zone of a semiconductor crystal.

It is yet another novel feature of the invention to combine a sensing coil means with a magnetic coil means to produce sonic oscillations of constant amplitude in a melt zone of a semiconductor crystal.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial elevated sectional view of an apparatus embodiment constructed in accordance with the principles of the invention; and

FIG. 2 is a similar view of another apparatus embodiment constructed in accordance with the principles of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The invention provides a means of producing semiconductor crystal rods having a radially uniform resistance characteristic which is relatively simple and capable of yielding reproducible results.

Generally, the invention comprises subjecting a stock semiconductor crystal rod to a floating melt zone purification and subjecting the melt zone to axially directed sonic oscillations. In this manner, an extremely good mixing action of the melt is always assured. The matter within the melt zone is homogeneously distributed throughout the zone so that upon solidification thereof, a uniform or substantially uniform radial resistance curve through a cross-section of such a crystal is attained.

Substantially all semiconductor materials can be subjected to the beneficial processing of the invention. Silicon is an extremely common semiconductor material and a stock crystal thereof is readily processed in accordance with the invention to attain radially uniform resistance characteristics.

Sonic oscillations are transmitted to a melt zone from a sound generating means and cause-mixing of matter within the melt zone so that a substantially homogeneous mass results in the resolidified crystal rod. A preferred sound generator means comprises a magnetic coil means that is energized by an alternating electrical current. The alternating current is preferably in the sound frequency range, such as 50Hz. Higher frequency alternating current (ultrasonic) may also be uti lized.

In an exemplary embodiment of the invention, the sonic oscillations are transmitted to a melt zone by a liquid body, such as an incompressible fluid, i.e., an oil, enclosed in a cylindrical container that is connected at one end to a sonic generator means and at the other end to a crystal mounting means of a floating melt zone device.

In another exemplary embodiment, sonic oscillations are transmitted to a melt zone by a metal body, such as a lightweight metal tube, for example composed of aluminum. The metal body is connected at one end to the sonic generator means and at the other end to a crystal mounting means.

In certain embodiments, the sound generator means includes sensing coil means positioned in working relationship therewith so as to maintain a constant amplitude of sonic oscillations. Preferably, the sensing coil comprises a piezoelectric sensing coil means, however, magneto-strictive coils or wire-strain gauges are also useful.

The invention was developed from the theory that matter withina solution or a melt is moved by a sonic oscillation passing through the solution. Such movement must cause location changes of the respective matter particles if the solution or melt is not enclosed in a constant volume container. This theory was confirmed by a simple experiment. A slightly alkaline solution was placed in a pair of paraffin-coated paper cups and a few granules of an alizarin dye (an anthraquinone) were added. One cup was immediately placed into the cone of a loudspeaker of about 0.3 watts, which was coupled with an ac. magnetic coil energized with a SOl-lz current. The other cup was placed in a quiet location. At first, noticeable striations were observed in both cups. After 2 minutes, the solution in the cup that was subjected to sonic oscillation was much better mixed than the quiescent solution. After three minutes (from the start of the test) a complete mix of the dye was observed in the solution subjected to sonic oscillations, whereas the quiescent solution exhibited a very uneven mixing, as evidenced by stationary dye areas or rings.

This theory is readily applied to crystal production from a melt thereof and produces an improved p-gradient therein because of the improved mixing of matter within the melt. In addition, the sonic oscillations promote the transfer of impurities and the like to the surface of a melt so that an increased purification takes place. In this manner, an improved crystal quality is achieved.

The principles of the invention can be advantageously combined with the methods described in German Letters Pat. Nos. 1,218,404 and 1,263,698. In accordance' with these patents, the rotatable crystal mounting means that supports a seed crystal or a stock crystal is shifted sideways relative to the induction heating coil. As will be appreciated, the normal arrangement of crystal mounting means in these type of devices is along a straight line that is concentric with the encompassing heating means. The combination of the aforesaid suggestions with those of the invention yields an additional mixing effect on a melt and further improves the crystal quality of the so-produced semiconductor crystals.

A yet further improved mixing action of the matter within a melt is achieved by rotating at least one of the crystal mounting means in an uneven or changing orbit relative to the other mounting means.

In thedrawings, like reference numerals denote similar portions. FIG. 1 illustrates an exemplary embodiment of an apparatus for practising the invention. A typical melt zone purification apparatus includes a housing which is evacuable, mounting means extending from opposing walls thereof for supporting a stock crystal rod, a heating means, such as an induction coil that encompasses a portion of the rod so as to define a melt zone thereon, suitable energy sources, means for selectively moving the rod within the housing, etc. For sake of brevity, all such conventional structure has been omitted from the drawing and will be referred to in the specification and claims as a floating zone operational environment F2 As shown, an upper wall 6 of the operationall environment FZ is provided with an opening for admitting a sound transmission'means 3 therein. An upper crystal mounting means 2 is positioned within the operational environment FZ to support one end of, for example, a silicon stock crystal rod 1. Of course, the other end of rod 1 is similarly supported and a melt zone is established between such ends. Mounting means 2 is connected with a means 21 for generating sonic oscillations via the sound transmission means 2. The transmission means 2 includes a tubular body 5 having a pair of opposing elastic diaphragm or bellow means 8a and 8b with a liquid 4 therebetween. The liquid 4 is generally incompressible and is bubble-free. An exemplary liquid is a low viscosity silicon oil. As indicated, body 5 thus comprises a liquid-filled hollow tube having suitable flanged ends for supporting elastic diaphragm members that are in direct contact with a means generating sonic oscillations and a means supporting the crystal.

The means 21 for generating sonic oscillations is illustrated as a magnetic coil means 9, which when energized by a suitable alternating current produces sonic oscillations that are transferred to transmission means 3. Because of the interconnection of parts, such sonic oscillations travel in an axial direction through the stock crystal 1 to produce a mixing action at the melt zone thereof.

In the embodiment shown, a sensing coil means 10 is provided in working relation with magnetic coil means 9 in order to maintain a constant oscillation amplitude. For example, coil means 10 may be interconnected with the ac. generator means energizing coil means 9 in a feed-back control loop or to an appropriate signal means (not shown) that indicates to an operator the oscillation frequency being produced and allows him to make a proper-adjustment. The sensing coil means 10 may comprise a piezoelectric sensing device, a magneto-strictive device or a wire-strain gauge that interconnects the magnetic coil means 9 with the transmission means 3 to maintain a constant oscillation'frequency as by producing a signal which allows an operator to adjust the frequency of the current being fed to the coil means 9.

The coil means 9 includes magnetic pole pieces 11, a wound induction coil 9a, a permanent magnet 12 and a means for contacting a source of alternating current and conducting such current to the coil 9a. The sensing coil means 10 includes a permanent magnet ring 13, magnetic pole pieces 14 and a means of transmitting a signal. The current source (not illustrated) provides an alternating electrical current to the coil 9 by a sliding contact as with a brush-contact means or with slip rings. For the sake of convenience, the mechanical drive means for moving the crystal mounting means 2 is also not illustrated.

During operation, the liquid 4 is caused to axially vibrate by coil means 9 so that the vibrations or oscillations are transferred via diaphragm means 8b to the stock crystal rod 1 and thus to the melt zone thereof. The direction of oscillation is indicated by the doubleheaded arrow 15.

FlG. 2 illustrates another embodiment for practicing the invention in the above-described operational environment F2 As shown, an upper crystal mounting means 2 supports an end of a stock silicon crystal rod 1. Mounting means 2 is connected to a sonic generator means 21 via a sound transmission means 3.

In this embodiment, means 3 comprises a metallic body composed of an outer tube 16 and an inner tube 17. The inner tube 17 is directly connected with a tlexible diaphragm or bellow means 8b and coaxially spaced from tube 16 by appropriate mounting means 18, such as Teflon (polytetrafluoroethylene) bushings.The tube 17 is composed of a lightweight metal, preferably aluminum. The sound transmission means 3 extends through an upper wall 6 of the operational environment F2 and is sealed at wall 6 by a vacuum-tight seal means 7. The upper end of the thin aluminum tube 17 is directly connected with a magnetic coil means 9, while the lower end thereof is connected with the crystal mounting means 2. Tube 17 is thus capable of axial movement relative to tube 16 and transmits the oscillations produced by generator21 to the crystal 1.

As shown, the coil means 9 includes magnetic pole pieces 11, a permanent magnet ring 12 and means for contact with a source of alternating current, such as slip-rings or the like. The mechanical drive means for moving the crystal mounting means 2 during the operation of the floating melt zone operational environment P2 is again not illustrated. During operation, the magnetic coil means 9 is energized by an alternating current and a portion thereof moves up and down at a given frequency. Such oscillations are transferred via tube 17 and mounting means 2 to the stock crystal rod 1 and thus to the melt zone thereof. The direction of oscillation is diagrammatically shown by the doubleheaded arrow 15.

A transmission means 3 comprised of tubes 16 and 17 can be connected in working relation with a cooling means. For example, the transmission means 3 can be filled with the cooling fluid in a conventional manner for cooling tube 17. Such a transmission means can be attached to a heat-exchange fluid-circulating means (not shown) for maintaining a select temperature within the transmission means. In accordance with the principles of the above-described combination, the device described in German Pat. application No. 1,519,891 (assignees case number VPA 66/1357) can be advantageously utilized in the practice of the invention, with improved temperature control.

As is apparent from the foregoing specification, the present invention is susceptible of being embodied with various alterations and modifications which may differ particularly from those that have been described in the preceding specification and description. For example, the mounting means 2 may be connected for eccentric rotational movement. For this reason, it is to be fully understood that all of the foregoing is intended to be merely illustrative and is not to be construed or interpreted as being restrictive or otherwise limiting of the present invention, excepting as it is set forth and defined in the hereto-appended claims.

1 claim as my invention:

1. In a method of producing a monocrystal semiconductor rod having a substantially uniform radial resistance characteristic by the crucible-free zone melting process which includes supporting a stock semiconductor rod at its ends by a pair of substantially vertically aligned opposing rod mounting means, generating a melt zone on an annular section of said rod by an annular induction heating means surrounding said rod, and moving said melt zone along the length of said rod from one end thereof to the other end, the improvement comprising:

generating sonic vibrations in a longitudinal direction substantially parallel to the axis of said rod by a magnetic coil activated with an alternating current; and

transmitting said axially directed sonic vibrations to at least one of said pair of rod mounting means.

2. In a method as defined in claim 1 wherein said sonic vibrations are generated by a magnetic coil acti- LII vated with an alternating current in an audio-frequency range of about 50 Hz.

3. In a method as defined in claim 1 wherein said sonic vibrations are transmitted to one of said pair of rod mounting means via a column of liquid.

4. In a method as defined in claim 1 wherein said sonic vibrations are maintained at a constant amplitude via a sensing coil.

5. In a method as defined in claim 1 wherein said sonic vibrations are maintained at a constant amplitude via a piezo-sensing device.

6. In a method as defined in claim 1 wherein said sonic vibrations are maintained at a constant amplitude via a wire-strain gauge.

7. In a method as defined in claim 1 wherein said sonic vibrations are transmitted to one of said pair of rod mounting means via a metal tube.

8. An apparatus for producing a substantially uniform radial resistance characteristic in a semiconductor crystal rod comprising:

a housing having a hollow chamber therein;

a pair of substantially vertically aligned opposing rod mounting means located within said chamber and spaced apart from one another for supporting a semiconductor rod therebetween;

a heating means located within said chamber so as to annularly surround a rod supported by said rod mounting means;

a magnetic coil means located outside said chamber and adapted to be excited by an alternating current and producing sonic vibrations in adirection substantially parallel to the axis of a rod supported by said rod mounting means; and

a transmission means having one end thereof in contact with said magnetic coil means and another end thereof in contact with one of said rod mounting means for transmitting sonic vibrations from said coil to said rod mounting means, said transmission means comprising a hollow tubular body having a pair of opposing elastic diaphragm means mounted at opposite ends thereof and a column of liquid between said diaphragm means.

9. An apparatus as defined in claim 8 including a magnetic sensing coil means in operative relation with said magnetic coil means for maintaining the sonic vibrations produced thereby at a constant amplitude.

10. An apparatus for producing a substantially uniform radial resistance characteristic in a semiconductor crystal rod comprising:

a housing having a hollow chamber therein;

a pair of substantially vertically aligned opposing rod mounting means located within said chamber and spaced apart from one another for supporting a semiconductor rod therebetween;

a heating means located within said chamber so as to annularly surround a rod supported by said rod mounting means;

a magnetic coil means located outside said chamber and adapted to be excited by an alternating current and producing sonic vibrations in a direction substantially parallel to the axis of a rod supported by said rod mounting means; and

a transmission means having one end thereof in contact with said magnetic coil means and another end thereof in contact with one of said rod mounting means for transmitting sonic vibrations from said coil to said rod mounting means, said transmit- 7 8 ting means comprising a pair of hollow metal tubes outer tube of said pair of tubes so that said inner coaxially mounted with one another, an inner tube tube is axially movable relative to said outer tube. of said pair of coaxially mounted tubes having a first end in contact with said magnetic coil means 11. An apparatus as defined in claim 10 including a and a second end in contact with one of said rod 5 cooling means connected in working relation with said mounting means, and an elastic diaphragm means transmission means. interconnected between said inner tube and an 

1. IN A METHOD OF PRODUCING A MONOCRYSTAL SEMICONDUCTOR ROD HAVING A SUBSTANTIALLY UNIFORM RADIAL RESISTANCE CHARACTERISTIC BY THE CRUCIBLE-FREE ZONE MELTING PROCESS WHICH INCLUDES SUPPORTING A STOCK SEMICONDUCTOR ROD AT ITS ENDS BY A PAIR OF SUBSTANTIALLY VERTICALLY ALIGNED OPPOSING ROD MOUNTING MEANS, GENERATING A MELT ZONE ON AN ANNULAR SECTION OF SAID ROD BY AN ANNULAR INDUCTION HEATING MEANS SURROUNDING SAID ROD, AND MOVING SAID MELT ZONE ALONG THE LENGTH OF SAID ROD FROM ONE END THEREOF TO THE OTHER END, THE IMPROVEMENT COMPRISING; GENERATING SONIC VIBRATIONS IN A LONGITUDINAL DIRECTION SUBSTANTIALLY PARALLEL TO THE AXIS OF SAID ROD BY A MAGNETIC COIL ACTIVATED WITH AN ALTERNATING CURRENT; AND TRANSMITTING SAID AXIALLY DIRECTED SONIC VIBRATIONS TO AT LEAST ONE OF SAID PAIR TO ROD MOUNTING MEANS.
 2. In a method as defined in claim 1 wherein said sonic vibrations are generated by a magnetic coil activated with an alternating current in an audio-frequency range of about 50 Hz.
 3. In a method as defined in claim 1 wherein said sonic vibrations are transmitted to one of said pair of rod mounting means via a column of liquid.
 4. In a method as defined in claim 1 wherein said sonic vibrations are maintained at a constant amplitude via a sensing coil.
 5. In a method as defined in claim 1 wherein said sonic vibrations are maintained at a constant amplitude via a piezo-sensing device.
 6. In a method as defined in claim 1 wherein said sonic vibrations are maintained at a constant amplitude via a wire-strain gauge.
 7. In a method as defined in claim 1 wherein said sonic vibrations are transmitted to one of said pair of rod mounting means via a metal tube.
 8. An apparatus for producing a substantially uniform radial resistance characteristic in a semiconductor crystal rod comprising: a housing having a hollow chamber therein; a pair of substantially vertically aligned opposing rod mounting means located within said chamber and spaced apart from one another for supporting a semiconductor rod therebetween; a heating means located within said chamber so as to annularly surround a rod supported by said rod mounting means; a magnetic coil means located outside said chamber and adapted to be excited by an alternating current and producing sonic vibrations in a direction substantially parallel to the axis of a rod supported by said rod mounting means; and a transmission means having one end thereof in contact with said magnetic coil means and another end thereof in contact with one of said rod mounting means for transmitting sonic vibrations from said coil to said rod mounting means, said transmission means comprising a hollow tubular body having a pair of opposing elastic diaphragm means mounted at opposite ends thereof and a column of liquid between said diaphragm means.
 9. An apparatus as defined in claim 8 including a magnetic sensing coil means in operative relation with said magnetic coil means for maintaining the sonic vibrations produced thereby at a constant amplitude.
 10. An apparatus for producing a substantially uniform radial resistance characteristic in a semiconductor crystal rod comprising: a Housing having a hollow chamber therein; a pair of substantially vertically aligned opposing rod mounting means located within said chamber and spaced apart from one another for supporting a semiconductor rod therebetween; a heating means located within said chamber so as to annularly surround a rod supported by said rod mounting means; a magnetic coil means located outside said chamber and adapted to be excited by an alternating current and producing sonic vibrations in a direction substantially parallel to the axis of a rod supported by said rod mounting means; and a transmission means having one end thereof in contact with said magnetic coil means and another end thereof in contact with one of said rod mounting means for transmitting sonic vibrations from said coil to said rod mounting means, said transmitting means comprising a pair of hollow metal tubes coaxially mounted with one another, an inner tube of said pair of coaxially mounted tubes having a first end in contact with said magnetic coil means and a second end in contact with one of said rod mounting means, and an elastic diaphragm means interconnected between said inner tube and an outer tube of said pair of tubes so that said inner tube is axially movable relative to said outer tube.
 11. An apparatus as defined in claim 10 including a cooling means connected in working relation with said transmission means. 